About 20 years ago I searched for that question. After it's first 30+ years, one early 70's body shop repair when it was tapped in the rear, etc., that part was looking bad on mine. I found no documented answer or original photos but I did examine a local "perfect" one-owner, professionally restored, multiple POCI gold award winner Verdoro '68 RA1 (Also featured in the Sept. 2010 Pontiac Enthusiast). It showed the natural metal for that part. I then cleaned up that part on my Solar Red '68 to match.
I used to be indecisive. Now I'm not sure. I feel like I am diagonally parked in a parallel universe. 1968 400 convertible (Scarlet) 1976 T/A - 455 SE (No Burt) 1968 400 coupe - R/A clone (Blue Pearl) (for sale) 1968 350 - 4 speed 'vert - 400 clone (the Beast!) 1968 350 convertible - Wife's car now- 400 clone (Aleutian Blue) (Blue Angel) 2008 Durango - DD 2008 GXP - New one from NH is AWESOME! 2017 Durango Citadel - Modern is nice! HEMI is amazing! 1998 Silverado Z71 - Father-daughter project 1967 326 convertible - Sold 1980 T/A SE Bandit - Sold